1. Field of the Invention
The present invention relates to a method of producing middle distillate products, such as kerosene, gas oil, etc., from the heavy distillate of petroleum distillate by two-stage hydrocracking, an apparatus used in the same hydrocracking, and a method of regenerating catalyst with reduced activity that has been used in the two-stage hydrocracking.
2. Description of Related Art
Hydrocracking is used in order to obtain middle distillate products for which there is a great demand, such as kerosene, gas oil, etc., from heavy oils, such as tar oil, residue, etc., for which there is relatively little demand. Related conventional technology is discussed in the book xe2x80x9cHydrocracking Science and Technology,xe2x80x9d by Julius Scherzer and A. J. Gruia, published by Mercel Dekker, Inc. (1996). According to this book, various hydrocracking processes are known, and the two-stage hydrocracking process is known as one of these processes.
Catalysts formed by containing a hydrogenation active component, such as molybdenum, tungsten, platinum, etc., in a carrier made of a refractory oxide, such as alumina, silica, silica alumina, zeolite, etc., are often used in two-stage hydrocracking. It is known that of these, hydrocracking catalysts containing zeolite have high cracking activity and stability, but generally have a tendency toward a lower middle distillate products selectivity when compared to hydrocracking catalysts that do not contain zeolite. Moreover, it is known that while hydrocracking catalysts that use noble metals, such as palladium and platinum, etc., as the hydrogenation active component provide high cracking activity and high middle distillate products selectivity, they are readily deactivated as a result of poisoning by sulfur compounds and they are difficult to use as catalysts in reaction zones where catalyst poisons are also present in high concentrations. It is further known that organonitrogen compounds and ammonia are poisons of hydrocracking reactions.
The concentration of organonitrogen compounds, ammonia, organosulfur compounds, and hydrogen sulfide in a feed oil fed to the second stage of the two-stage hydrocracking process is very low. Although strong cracking reactivity is easily obtained under such reaction conditions, over-cracking readily occurs and it is difficult to obtain a high middle distillate products yield. A method of adjusting operating conditions is disclosed in U.S. Pat. No. 3,213,013 whereby a nitrogen-containing base, such as an amine or ammonia, is introduced to the feed oil of the hydrocracking process in order to inhibit over-cracking during the initial stage of the operation and the amount of the nitrogen-containing base introduced is adjusted in accordance with the extent of catalyst degradation in order to obtain gasoline with a constant high octane value. A method for improving middle distillate selectivity is disclosed in U.S. Pat. No. 3,816,296 wherein ammonia or an amine is introduced during operation of the hydrocracking process. U.S. Pat. No. 5,141,909 and U.S. Pat. No. 5,366,615 disclose improvement of selectivity of jet fuel distillate by addition of ammonia or ammonia precursor during hydrocracking with a catalyst containing noble metal and zeolite. A method is disclosed in U.S. Pat. No. 5,888,377 wherein system operation can be made stable by introducing ammonia to the second-stage process with a catalyst containing zeolite during start-up of the two-stage hydrocracking process.
Moreover, ammonia TPD (ammonia temperature programmed desorption) is widely known as an effective method of characterization of the acidity of zeolite. For instance, C. V. Hidalgo et al. described in the Journal of Catalysis, volume 85, pages 362 to 369 (1984) that distribution of the amount of Bronsted acid center and acid strength of Bronsted acid center can be determined by ammonia TPD. Japanese Patent Application Laid-Open No. Hei 3-212494 (U.S. Pat. No. 4,894,142) discloses hydrocracking catalysts suitable for selective production of middle distillate products. The catalysts use hydrogen-Y-type zeolite with a low acidity whose unit lattice constant is 24.20 to 24.40 xc3x85 and amount of ammonia TPD acid as determined by ammonia TPD is less than 2.00 mmol/g.
Catalysts with reduced activity that have been used in hydrocracking processes are mostly regenerated after being removed from the reaction vessel or while being left packed in the reaction vessel. The regenerated catalyst reused as hydrocracking catalysts. Hydrocracking operation often is interrupted for the regeneration process of the catalyst, but this regeneration process wastes time in the hydrocracking process and is therefore undesirable from an economical point of view.
The obtained middle distillate products must satisfy specific properties as petroleum products. For example, Kazuo Yuta discloses in Koku Gijutsu, No. 501, 17-23 (1996) that combustibility of jet fuel is good on the order of paraffin, naphthene, and aromatics and that combustion performance can be assessed based on the smoke point, which is related to the aromatic content. Hiromichi Ikebe discloses in Petorotekku, volume 17, pages 1032 to 1037 (1994) that the cetane number is an index of ignitability of gas oil for diesel engines and that the cetane number of paraffins is high and of naphthalenes is low. Moreover, the cetane index is known as an index closely related to the cetane number and is widely used as an index of the quality of gas oil. These facts show that jet fuel and gas oil for diesel engines comprising a high aromatic content are undesirable because fuel properties degrades. Accordingly, it is predictable that inferior fuel properties likely results in products from jet fuel and gas oil for diesel engines produced from feed oil with a high aromatic content.
An object of the present invention is to provide a two-stage hydrocracking method for producing middle distillate products, with which the conversion percentage is high and the cracking reaction of the entire two-stage hydrocracking process proceeds with long-term stability at high selectivity for middle distillate products, and a hydrocracking apparatus used for the method thereof. In particular, the present invention is directed to provide a method for producing high-quality intermediate distillates that meet environmental regulations and have excellent properties as petroleum products, such as fuel oil, etc., by conversion of lower quality starting materials. Futhermore, the present invention provides a catalyst regeneration method with little time wasted during hydrocracking process and high operation flexibility.
The inventors of the present invention performed research for methods of producing middle distillate products by two-stage hydrocracking including at least two steps, processes in each of which crude hydrocarbon oil is brought into contact with a hydrogenation catalyst in the presence of hydrogen to obtain hydrocracking oil. The inventors also performed research for a catalyst that is appropriate for the above-mentioned two-stage hydrocracking. Moreover, the inventors have achieved the present invention based on a conception regarding a novel method of adjusting the catalysts to be used in a first-stage and a second-stage and the reaction environment with emphasis on the fact that while catalyst poisons are present in high concentrations during the first-stage, catalyst poisons are present in very low concentrations during the second-stage.
In accordance with the first aspect of the present invention, there is provided a method of producing middle distillate products by two-stage hydrocracking comprising the steps of:
bringing a first-stage feed oil containing a hydrocarbon component with a boiling point of 316xc2x0 C. or higher into contact with a first-stage catalyst in the presence of hydrogen to obtain a first-stage product;
separating the first-stage product into a heavy component and a light component containing the middle distillate products;
bringing a second-stage feed oil containing the heavy component of the first-stage product into contact with a second-stage catalyst in the presence of hydrogen to obtain a second-stage product; and
separating the second-stage product into a heavy component and a light component containing the middle distillate products and bringing part of the heavy component separated from the second-stage product into contact with the second-stage catalyst again;
wherein a hydrocracking activity of the first-stage catalyst is greater than a hydrocracking activity of the second-stage catalyst.
The present invention is characterized in that cracking activity of the first-stage catalyst used in the reaction zone in which catalyst poisons are present in high concentrations is controlled to be higher than that of the second-stage catalyst used in the reaction zone in which catalyst poisons are present in very low concentrations. This characteristic has the effect of prolonging life of the catalyst that is used in the reaction zone where catalyst poisons are present in high concentrations and also has the effect of improving middle distillate products yield by inhibiting over-cracking in the reaction zone in which catalyst poisons are present in very low concentrations.
In order to obtain the necessary catalytic cracking activity in the present invention, it is preferred that the first-stage catalyst is formed with a hydrogenation active component supported on a carrier containing zeolite and the second-stage catalyst is formed with a hydrogenation active component supported on a carrier that contains less zeolite than the first-stage catalyst. Alternatively, the first-stage catalyst may be formed with hydrogenation active component supported on a carrier containing zeolite and the second-stage catalyst may be formed with hydrogenation active component supported on a carrier that contains zeolite with a lower amount of ammonia TPD acid than the zeolite which is contained in the carrier of the first-stage catalyst. Preferably the first-stage catalyst and second-stage catalyst are formed with non-noble metal hydrogenation active components supported on a carrier consisting of refractory oxide, including zeolite.
Preferably, the second-stage catalyst of the present invention has 0.01 wt % or more nitrogen content. This has the effect of bringing the reaction to a stable state during the initial stage operation under hydrocracking conditions and the effect of increasing cracking activity when operation has entered the stable state after the initial stage. In order to add nitrogen component to the second-stage catalyst, the second-stage catalyst is preferably brought into contact with a pre-treatment agent selected from ammonia, organonitrogen compound, or petroleum distillate products containing 10 ppm or more nitrogen content before being used under hydrocracking conditions. Additive selected from ammonia, organonitrogen compound, or petroleum distillate product comprising 10 ppm or more nitrogen content may also be further brought into contact with the catalyst under hydrocracking conditions when the second-stage catalyst is used under hydrocracking conditions.
According to the second aspect of the present invention, there is provided a method of producing middle distillate products by two-stage hydrocracking using a first-stage catalyst and a second-stage catalyst layers, each of which has a catalyst is formed by supporting non-noble metal hydrogenation active component on a carrier, comprising:
a first step of regenerating a catalyst in one of the first-stage and second-stage catalyst layers by heating and exposing to an oxygen atmosphere the catalyst and by bringing the catalyst in the one of the catalyst layers into contact with a sulfur compound; and
a second step of bringing feed oil into contact with the catalyst in the other of the first-stage and second stage catalyst layers in the presence of hydrogen during the first step in order to obtain a product. Since one of the first-stage catalyst and the second-stage catalyst can be regenerated while hydrocracking is continued with the other catalyst and therefore, it is not necessary to stop or interrupt the entire hydrocracking apparatus during catalyst regeneration. Regeneration of the first-stage catalyst and regeneration of the second-stage catalyst can also be performed in succession.
According to the third aspect of the present invention, there is provided a two-stage hydrocracking apparatus for producing middle distillate products, comprising:
a first-stage catalyst layer having a first-stage catalyst, which hydrocracks a first-stage feed oil containing a hydrocarbon component in the presence of hydrogen to produce a first-stage product;
a first-stage separator which separates the first-stage product into a heavy component and a light component comprising the middle distillate products;
a second-stage catalyst layer having a second-stage catalyst with a lower activity than the first-stage catalyst, the second-stage catalyst hydrocracking second-stage feed oil containing a hydrocarbon component in the presence of hydrogen to produce a second-stage product;
a second-stage separator which separates the second-stage product into a heavy component and a light component containing the middle distillate products; and
a conduit which is provided between the second-stage catalyst layer and the second-stage separator and recycles part of the heavy component separated from the second-stage product to the second-stage catalyst layer.